Right now I don't think there is any form of "active" spindle. There is the primary spindle which responds to commands with no spindle designation or $0 commands. And all spindle commands intended for spindles other than the primary one must be appended with with $n, where n=the spindle's designation number.
Before the addition of multiple spindle support, one of the main work arounds was to create custom M-codes to designate "active" spindles that would then receive the standard spindle commands. But that isn't how the current multi-spindle support works. In some ways I think that is almost a better way to make it work. And It still can be done this way, but it does make for a much more complicated hal configuration.
